1. Field
The present embodiments relate generally to communications, and more specifically, to radio frequency receivers in communications.
2. Related Art
Radio frequency (RF) receivers used in modern communication systems may support multiple modes and networks, such as 3G wideband code division multiple access (WCDMA) and 2G Global System for Mobile communications (GSM) using time division multiple access (TDMA). A received RF signal in a certain mode may be translated into a specified frequency band and processed to retrieve the information contained in the RF signal. For example, an RF receiver may amplify, filter, and mix an analog received RF signal into in-phase (I) and quadrature (Q) signals that may be converted into digital signals for further processing.
The multiple modes may utilize the same RF receiver architecture but have different frequency ranges and linearity, noise figure, and sensitivity requirements. Noise figure is a measure of degradation of a signal to noise ratio caused by components in the RF receiver. In particular, a WCDMA system has full duplex functionality where separate receive and transmit signals may be active simultaneously. A higher power transmitted signal may leak into the receive signal in such a system. To relax linearity and noise requirements due to the transmit signal leakage, a conventional WCDMA system may include a surface acoustic wave (SAW) filter prior to a mixer and downstream processing stages. In addition, a conventional WCDMA system may include a low noise amplifier (LNA) to meet sensitivity, noise figure, and gain control requirements. Using a SAW filter, LNA, amplifiers, and other components may result in increased die size, current drain, cost, and complexity, and reduced performance of an RF receiver.
A mixer may combine the received RF signal with the I and Q pulses. The I and Q pulses may be generated by a pulse generator based on a periodic signal from a local oscillator. The pulse generator in a conventional RF receiver may generate pulses with a 150% duty cycle, including I pulses 0 and 180 degrees out of phase from the periodic signal, and Q pulses 90 and 270 degrees out of phase from the periodic signal. The I and Q pulses may control switches in the mixer to combine the received RF signal with the I and Q pulses. The switches may not be on at the same time if the rise and fall times of the I and Q pulses are ideal. However, if the 50% duty cycle I and Q pulses have non-ideal rise and fall times, the switches in the mixer may be partially on when an amplitude of the I and/or Q pulse is above a crossover threshold, resulting in degraded performance and increased current drain.
Large transmit signal leakages may also affect the gate voltage bias of a mixer, leading to degraded noise figure, 1/f noise, gain, and second order input intercept point (IIP2). Moreover, a conventional RF receiver including operational amplifiers and voltage mode mixers may have poor IIP2 and third order input intercept point (IIP3). A conventional RF receiver may also include transimpedance amplifiers that may have a pole in the modulation bandwidth. Such transimpedance amplifiers may increase current drain, integrated noise, and die area.